As a conventional method of utilizing a semiconductor material that acts as a photocatalyst, there is known a technique for obtaining hydrogen through water decomposition or for obtaining electrical energy by irradiating the semiconductor material with light (see, for example, Patent Literatures 1 and 2).
Patent Literature 1 discloses a water decomposition semiconductor photoelectrode having a structure in which a photocatalyst and a solar cell are laminated together, and a water decomposition system using this electrode. This semiconductor photoelectrode is composed of a photocatalytic film, a transparent conductive film, a transparent substrate provided with an electrode for electrically connecting the front and back surfaces thereof, a transparent conductive film, an electrolyte solution, a dye-supported titanium oxide layer, a metal substrate, and a hydrogen evolution catalytic layer, in this order from the light receiving surface side. Patent Literature 1 discloses that water is decomposed by irradiating this semiconductor photoelectrode with sunlight to obtain hydrogen gas and oxygen gas. Specifically, Patent Literature 1 describes the use of a film made of a material selected from the group consisting of titanium oxide, tungsten oxide, and iron (III) oxide, as the photocatalytic film.
Patent Literature 2 discloses a regenerative photoelectrochemical cell including a polycrystalline metal oxide semiconductor as a photocatalyst. Patent Literature 2 discloses a photoelectrochemical cell whose polycrystalline metal oxide semiconductor does not corrode and which has an improved electrical energy yield in the visible spectral region (more specifically, in the spectral region of sunlight), and the use of the cell.